Volume control is almost always required for compressors, as the volume output produced by the compressor hardly ever equals the demand, and the excessive pressurized gas is not to escape uselessly into the atmosphere. The volume may be influenced by altering the speed of the compressor. Practically, this method is limited to those compressors which are used with combustion motors, because in electric operation, mostly rotary current motors with fixed number of revolutions are used. Compressors driven by a constant number of revolutions, therefore, require special control devices.
Control devices for continuous regulation of volume output of screw compressors have been disclosed (West German Pat. Nos. DT-OS 1,628,382, DT-OS 1,628,385) where slide valve control is utilized. This device consists mainly of a control slide valve arranged between the engaging helical rotors. The slide valve is designed with the same cross section as the housing. When moving the slide valve in the direction of the outlet, a space is freed through which the gas already aspirated may flow back to the suction side. The further the slide valve is opened the more the actually aspirated gas quantity decreases. The control edge of the control slide valve is also moved towards the pressure side so that the built-in pressure ratio is maintained essentially constant for a sufficiently large control area. The control slide valve is actuated pneumatically, hydraulically, or mechanically.
Partial load conditions of the described continuous slide valve control are shown on the enclosed drawing (FIG. 1) by the line drawn between points 1 and 2. Compared with the ideal line which runs as a straight line between points 1 and 6 and which indicates the least operating requirements for the decreased volume output, there is some deviation, which becomes more obvious with decreasing percentage figures. Relief is already provided for volume output flow of less than 20% approximately by discharging into the atmosphere or by decompression of compressed gas in the suction line. When gas counter pressure is eliminated and the control slide valve is completely opened, i.e., without internal compression, operation requirements at zero delivery (the so-called idling) at point 3 only amounts to 10% approximately compared with the operation requirements at full delivery (point 1).
The control device described above, regulating the volume for screw compressors has not met with general success, being that it requires high construction expenditures making it too expensive.
Another known control device for continuous regulation of volume flow in screw compressors is described in DEMAG News, Issue 182, 1966, in an article titled "DEMAG Helical Compressor, Operation, Construction, Comparison with other compressor constructions, field of application, and special constructions". The control device consists mainly of a throttle provided in the suction line of the screw compressor. Regulation by means of a throttle is very wide-spread in screw compressors, as the control devices required for this hardly matter economically.
On the other hand, this type of control has a considerable disadvantage. Operating requirements decrease very little with decreasing volume output flow in throttle controls, as may be seen on the attached drawing (FIG. 1). This becomes particularly obvious when tracing the line (throttle regulation) between points 1 and 4 as opposed to slide valve regulation. When the throttle is completely closed, zero delivery of the compressor occurs. Operating requirements at point 4 still amount to 75% approximately as opposed to full load. When relieving the compressor by discharging into the atmosphere or by decompression in the suction line, i.e. elimination of gas counter pressure, operating requirements at point 5 result in 50% as opposed to full load, which, dependent upon the built-in pressure ratio, may also be higher. In general, however, operating requirements are always higher in throttle regulation than in valve regulation which is due to the fact that despite compressor relief the built-in pressure ratio is maintained, and a corresponding amount of compression work must be done.
For the above-mentioned slide valve, as well as throttle regulation, no special pressurized container is necessary for the continuous control of volume output flow. In the areas of partial load, however, the mentioned losses occur compared to the ideal line. Such losses can be eliminated by providing a pressurized vessel, in accordance with requirements, as a holding tank between the compressor and consumption, and operating only a full delivery (point 1), and idling (point 6). This so-called by-pass or standstill control offers maximum economy as in full delivery the compressor operates with specifically lowest operating requirements and in idling there are no losses. However, this type of regulation cannot be utilized at all times. Specifically, once the pressurized vessel volume is decreased in order to obtain a reasonable price of acquisition, more frequent switching operations (on and off) are entailed for the compressor components. The often-used electro-motors overheat so that by necessity operation between full (point 1) and zero (point 6) delivery is impossible. Instead, the motor continues to run and the compressor idles at zero delivery. For slide valve control and relief this applies to point 3, for throttle control and relief to point 5. In order to attain economical operation of a screw compressor provided with so-called continuous control, it is of the utmost importance that operating requirements, particularly for longer periods of zero delivery, are as low as possible.
It is the object of the invention to propose a screw compressor of the type mentioned initially, with control device to regulate volume output flow which, while being of simple construction, facilitates the lowest possible operating requirements for the compressor. This is solved by designing the helical part of at least one of the rotors -- given the axial clearance necessitated by the construction -- shorter than the axial distance between the end walls of the working chamber therefor and by making this shorter rotor axially displaceable. In accordance with this arrangement, one of the rotors of the screw compressor is displaced towards the suction side during operation, so that a free space is formed on the pressure side within the housing. A connection is made between this space and the suction side via gaps formed between the rotors. This facilitates zero delivery while idling without internal compression. The check valve required in the pressure line, in accordance with governing regulations, prevents the return of the compressed gas from the consumption side. When full delivery is to commence again, the rotors must be returned to their starting position. As internal compression is completely eliminated during zero delivery and idle running and sufficiently large overflow cross sections are opened toward the suction side, no compression losses occur and reverse flow losses are reduced to a minimum. Due to the small operating requirements in idle run, the control device is also suitable to assist in starting the compressor operation initially. During full delivery the effectiveness of the screw compressor is not impaired.
For structural reasons, it is advantageous to design the non-driven, i.e. female, rotor shorter. In accordance with another characteristic of this invention the shorter rotor is axially displaceable by means of a piston. For practical reasons, the piston is positioned in a cylinder connected to a pressure medium source. Another detail of the invention provides unilateral charge of the piston with a pressure medium and places the piston under the influence of a return spring.